The present invention relates to fiber optic couplers and more particularly to a method of fabricating a low loss fused biconical taper fiber optic coupler.
Several published articles have described the fabrication of relatively low loss fused biconical taper (FBT) fiber optic couplers. These articles are as follows: D. C. Johnson, B. S. Kawasaki and K. O. Hill, "Low Loss Reflection Star Couplers for Optical Fiber Distribution Systems", Applied Physics Letters, Vol. 35(7), pages 479-481, Oct. 1, 1979; E. G. Rawson, A. B. Nafarrate, "Star Couplers Using Fused Biconically Tapered Multimode Fibers", Electronic Letters, Vol. 14, No. 9, Apr. 27, 1978; and E. G. Rawson, M. D. Bailey, "Bitaper Star Couplers with Up to 100 Fibre Channels", Electronic Letters, Vol. 15, No. 4, July 5, 1979. According to these articles the FBT couplers are fabricated by grouping any number, N, of optical fibers over some length, L, and twisting and fusing the fibers together with a heat source while simultaneously pulling the fibers to form a biconical taper of N fibers. Both reflection and transmission star couplers can be made using this described method.
Typically, couplers used in fiber optic systems are required to have an even distribution of power among the output ports of the coupler. An even distribution of the power has not been demonstrated for FBT star couplers with acceptable insertion losses using the conventional fabricating technique above described. This disadvantage appears to be caused by a preferential recoupling of light back into the excited fiber of the coupler. The excited fiber, referred to hereinafter as the throughput fiber on the output end, will always have a higher power level than any of the tapoff fibers and this difference between tapoff and throughput fibers will depend on the number N of fibers in the coupler. For couplers with greater than eight fibers, the throughput fiber will have significantly more power (usually greater than 50%) than any of the tapoff ports.